1. Field of the Invention
The present invention relates to a method for producing an optical waveguide having a tapered part (hereinafter, referred to as a "tapered waveguide"), and in particular, to a method for producing an optical waveguide having a tapered part in an optical integrated circuit formed on a substrate.
2. Description of the Related Art
Recently, optical components are being more and more reduced in size and increased in the degree of integration. Processing technologies in the sub-micron order have been established. In conventional film formation or etching, it is routine to uniformize the processing rate of the sample, the film thickness, and the etching depth in a processed plane of the sample to a maximum possible extent. In other words, processing is performed in a plane parallel to the surface of the substrate of the sample so as to minimize non-uniformity in the processing rate of the plane.
In some cases, the film forming rate or the etching rate of the film on a certain part of the substrate is intentionally made different from the rest of the substrate so as to form a tapered part having a surface inclined with respect to the surface of the substrate. Among various devices produced using such a technology, the tapered part of the optical waveguide is effectively utilized for changing the path of light to the thickness direction of the substrate with no waste, or for allowing the light to pass with no waste through the border of a plurality of areas having different effective refractive indices.
FIG. 4A is a plan view of a mode splitter 41 (also referred to as "waveguide element") to which the tapered waveguide disclosed in Japanese Laid-Open Publication No. 6-82644 is applied. FIG. 4B is a cross sectional view thereof.
The waveguide element 41 includes two waveguide areas A and B having different thicknesses from each other, while the thickness of each of the waveguide areas A and B is uniform. The waveguide areas A and B are combined by a tapered part C having a surface which is inclined sufficiently slowly with respect to the wavelength of the light. The thickness of the tapered part C continuously changes from the thickness of the waveguide area A to the thickness of the waveguide area B.
TM mode light and TE mode light incident on the tapered part C at a certain incident angle are refracted to make an angle e therebetween due to the difference in equivalent refractive indices of the waveguide areas A and B.
The tapered part C of the waveguide element 41 can be formed by processing methods such as, for example, dry etching, wet etching, ion milling or machining. Alternatively, the tapered part C can be formed using shadow masking during formation of the waveguide element 41 using sputtering, vapor deposition, CVD (chemical vapor deposition) or the like. Shadow masking refers to covering an area of a layer and depositing another layer having a tapered part using particles which jump to the covered area.
In FIG. 4B, reference numeral 46 denotes a substrate having a refractive index of 1.47, reference numeral 45 denotes a buffer layer having a refractive index of 2.3, and reference numeral 44 denotes a waveguide layer having a refractive index of 1.52.
Hereinafter, formation of a tapered part using etching and shadow masking will be described.
First, production of a tapered part using etching will be described with reference to FIGS. 5A through 5I. The method illustrated in FIGS. 5A through 5I is disclosed in Japanese Laid-Open Publication No. 4-55802.
A Si substrate 51 shown in FIG. 5A is treated by thermal oxidation to form a first SiO.sub.2 film 52 on the Si substrate 51 as shown in FIG. 5B. On the first SiO.sub.2 film 52, a second SiO.sub.2 film 53 is formed as shown in FIG. 5C. A photoresist 54 is formed on the second SiO.sub.2 film 53 as shown in FIG. 5D, and then the photoresist 54 is patterned as shown in FIG. 5E.
Next, wet etching is performed using the patterned photoresist 54 as a mask as follows. The first and second SiO.sub.2 films 52 and 53 are etched using an appropriate etchant. The second SiO.sub.2 film 53, which has a higher etching rate than that of the first SiO.sub.2 film 52, is etched faster than the first SiO.sub.2 film 52. The first SiO.sub.2 film 52, which is below the second SiO.sub.2 film 53, is slowly etched. In detail, the first SiO.sub.2 film 52 is etched by the amount which is in proportion to the time period during which the first SiO.sub.2 film 52 is exposed to the etchant. An area of the first SiO.sub.2 film 52 which is not covered by the second SiO.sub.2 film 53 is etched deep, and an area of the first SiO.sub.2 film 52 which is covered by the second SiO.sub.2 film 53 is etched shallow. As a result, the etched part of the first SiO.sub.2 film 52 results in a tapered part 55 as shown in FIG. 5F.
After the photoresist 54 is removed as shown in FIG. 5G and the second SiO.sub.2 53 is also removed as shown in FIG. 5H, an optical waveguide 56 is formed on the substrate 51 so as to cover the first SiO.sub.2 film 52 as shown in FIG. 5I. The first SiO.sub.2 film 52 having the tapered part 55 at one end thereof functions as a buffer layer in an area for changing the propagation direction of light in the optical waveguide 56.
With reference to FIGS. 6A and 6B, the principle of forming a tapered part using shadow masking will be described. Shadow masking is disclosed in, for example, Journal of Lightwave Technology, Vol. 8, No. 4, pp. 587-593, (April, 1990). FIG. 6A shows a mask 61 disposed above a substrate 63, and FIG. 6B is a partial cross sectional view of the mask 61.
As shown in FIG. 6A, the metal mask 61 is held above the substrate 63 while a certain gap is kept between the mask 61 and the substrate 63 by a spacer 62. When film-forming particles are caused to jump from above the mask 61 toward the substrate 63, some particles enter the area covered by the mask 61. Thus, the thickness of a film 65 formed by the particles changes on the area of the substrate 63 right below the end of the mask 61. In detail, the film 65 has a tapered part 64, the thickness thereof decreasing toward the area covered by the mask 61. The film 65 is also formed on the mask 61.
The shape of the tapered part 64 is determined by, for example, the cross sectional shape of the mask 61, the distance between the mask 61 and the substrate 63, the size of the source of the particles, and the distance between the source and the substrate 63. Since the particles jump obliquely as well as vertically with respect to the surface of the substrate 63, the shape of the tapered part 64 covered by the mask 61 is unclear.
FIG. 7 shows another method for producing a tapered part using shadow masking. The method shown in FIG. 7 is disclosed by Japanese Laid-Open Publication No. 7-134216.
As shown in FIG. 7, a substrate 72 having a patterned photoresist 71 thereon is supported on a sample table 76 in an inclined manner. The sample table 76 is provided below a film formation particle source 75 of a film forming apparatus. When the substrate 72 is supported on the sample table 76 in the inclined manner, the substrate 72 is not lying against the sample table 76 which is cooled, and thus the substrate 72 is not sufficiently cooled. In order to compensate for such an inconvenience, a highly thermally conductive, right-angled triangular metal jig 74 having a surface inclined with respect to the surface of the sample table 76 is provided on the sample table 76, and the substrate 72 is provided on the metal jig 74 with a vacuum grease provided therebetween.
The patterned photoresist 71 is provided on a part of the substrate 72, so that an area of the substrate 72 which is not covered by the photoresist 71 is underneath the photoresist 71 with respect to the direction of the particle flow.
When film formation is performed in such a state, film-forming particles 79 jumping from the film formation particle source 75 enter a surface of the photoresist 71 formed on the substrate 72 obliquely. Thus, a film 77 formed on the substrate 72 has a tapered part on an area of the substrate 72 shadowed by the photoresist 71. On the photoresist 71 also, a film 78 is formed.
After the film formation, the photoresist 71 and the film 78 on the photoresist 71 are removed by lift-off, thus leaving the film 77 on the substrate 72. In detail, the photoresist 71 is dissolved by a solvent such as acetone or the like, and thus removed together with the film 78 thereon. As a result, the film 77 having the tapered part is obtained.
FIGS. 8A through 8H illustrate another method for producing a tapered part using shadow masking. The method shown in FIGS. 8A through 8H is disclosed by the U.S. Pat. No. 4,256,816.
By the method shown in FIGS. 8A through 8H, a shadow mask having three layers is used. The shadow mask includes, for example, two photoresist layers sandwiching an A1 layer therebetween.
As shown in FIG. 8A, a substrate 80 is coated with a bottom photoresist layer 82 and dried. Then, the entire surface of the assembly of the substrate 80 and the bottom photoresist layer 82 is exposed to light. Next, as shown in FIG. 8B, an A1 layer 84 having a thickness of 5 to 20 nm is formed on the bottom photoresist layer 82. The A1 layer 84 acts as a protective layer for preventing the bottom photoresist layer 82 from dissolving while a top photoresist layer 86 is formed.
As shown in FIG. 8C, the A1 layer 84 is coated with the top photoresist layer 86 and dried. Then, the top photoresist layer 86 is selectively exposed to light using a light blocking mask 88. Next, as shown in FIG. 8D, the top photoresist layer 86 is developed to form an opening 89 therein. Then, A1 layer 84 is treated by etching and the like, using the top photoresist layer 86 having the opening 89 as a mask. Thus, as shown in FIG. 8E, an opening 89a which is larger than the opening 89 is formed in the A1 layer 84. The bottom photoresist layer 82 is partially removed by an etchant through the opening 89a, thereby forming an opening 89b which is larger than the opening 89a in the bottom photoresist layer 82. A side wall 81 of the opening 89b in the bottom photoresist layer 82 is hidden below the top photoresist layer 86. Such a type of shadow mask is referred to as an "undercut-type" or "T-shaped" shadow mask.
By such a method, the opening 89a formed in the A1 layer 84 and the opening 89b formed in the bottom photoresist layer 82 are patterned by transfer of the pattern of the opening 89 formed in the top photoresist layer 86. Accordingly, formation of the openings 89a and 89b does not require any additional masks or positioning.
When forming a thin film 90 of metal or the like using the undercut-type three-layer shadow mask produced in the above-described manner, film-forming particles which have passed through the openings 89, 89a and 89b are deposited on the substrate 80 as shown in FIG. 8G. By removing unnecessary layers on the substrate 80, the metal thin film 90 having a desired pattern is obtained as shown in FIG. 8H. The metal thin film 90 has a trapezoidal cross section with two sides thereof being tapered.
Film formation performed by depositing film-forming particles, e.g., vapor deposition, sputtering, or CVD are advantageous in having a wider selection of materials and thus being more widely adaptable for optical waveguides of various specifications, when compared with film formation performed by thermal oxidation. However, SiO.sub.2 films produced by deposition of film-forming particles have such disadvantages compared with the SiO.sub.2 film formed by thermal oxidation that (1) grain boundaries are more easily generated, (2) the surface is generally rougher and more porous; and (3) the state of the tapered surface is deteriorated by etching. For example, the surface roughness is significantly increased by etching. Due to these disadvantages, a tapered waveguide formed by film formation performed by depositing film-forming particles has a greater optical loss than that of the tapered waveguide obtained by processing the SiO.sub.2 film formed by thermal oxidation.
According to the method illustrated by FIGS. 5A through 5I, after the second SiO.sub.2 film 53 used for controlling the etching rate is removed, a step portion is generated between the tapered part of the first SiO.sub.2 film 52 which is not covered by the second SiO.sub.2 film 53 and the flat area of the first SiO.sub.2 film 52 which was covered by the second SiO.sub.2 film 53, due to the difference in etching rate between the first SiO.sub.2 film 52 and the second SiO.sub.2 film 53. The step portion significantly affects the optical loss of the tapered waveguide.
According to the method illustrated by FIGS. 6A and 6B, the resultant tapered part obtained by this method is not sufficiently small to reduce the size of the optical integrated circuit including the tapered waveguide or to raise the degree of integration. An overhanging part 61a of the mask 61 has a length LH of about 1 mm or more, and a gap W between the overhanging part 61a and the substrate 63 is also 1 mm or more. Accordingly, the tapered part 64 has a length LT of several millimeters. Moreover, according to this method, attachment and detachment of the shadow mask to and from the substrate 63, and washing of the substrate 63, make mass production difficult.
According to the method shown in FIG. 7, the tapered part is formed only on an area of the substrate 72 which is shadowed by the photoresist 71. Moreover, the thickness and the refractive index of the film used in the tapered waveguide are not uniform within one optical waveguide or within one pattern area of the optical waveguide. Due to such non-uniformity, the number of substrates (wafers) which can be processed at one time is limited, and device characteristics vary from device to device. The method illustrated in FIG. 7 requires a special apparatus for cooling the substrate 72.
The method shown in FIGS. 8A through 8H involves the following problems.
Formation of the protective layer 84 in a vacuum state restricts the material selection. Even formation of the protective layer by spin-coating or other more productive methods in order to broaden the material selection is not appropriate because a protective material containing a solvent dissolves the bottom photoresist layer 82.
When the baking temperature of the bottom photoresist layer 82 is raised in order to make the bottom photoresist layer 82 more resistant against the solvent, developing and removal of the bottom photoresist layer 82 become difficult. The bottom photoresist layer 82 also restricts the method of formation of the protective layer in a vacuum state if the protective layer 84 is formed by, for example, sputtering; the reason being the property of the surface of the bottom photoresist layer 82 is changed by plasma and thus developing and removal of the bottom photoresist layer 82 become difficult.
Since the bottom photoresist layer 82 entirely exposed to light exists below the protective layer 84, optical reaction of the bottom photoresist layer 82 is promoted excessively during the baking of the top photoresist layer 86, causing foaming or delamination of the protective layer 84. It is difficult to provide a set of conditions (exposure conditions, baking conditions, resist material) for preventing such a phenomenon. Thus, the top and bottom photoresist layers 86 and 82 cannot be processed with satisfactory reproducibility.
As the material of the protective layer 84, A1 is optimum in consideration of suitability with the IC process. However, an alkaline solution, which is most often used today for developing the top photoresist layer 86, etches the A1 layer when developing the top photoresist layer 86. As a result, the shape of the shadow mask cannot be reproduced.